With freelancing web developer, chances of liability are very, very little. It's not like someone can trip over your website and break their foot. That said, be sure to have a good standard contract that you use to avoid clients being able to come back at you after the job is done.
... Basically, you need them to sign off and absolve you of further work once complete.
I would consult with local accountant though and they should be able to tell you when it makes sense to create an LLC. LLC's are more expensive to setup than a sole proprietorship and the accounting cost are higher. Where I am in Canada, setting up a corporation makes sense when you are making more than $60k profit ... mainly for tax reasons.
... A local accountant should tell you how that figures where you are.